Biodegradable resins can generally be classified into two types. One is the polymers or copolymers of hydroxy acids having the property of biodegradability which are synthesized through biological or chemical methods, such as poly 3-hydroxy butyrate (PHB). The advantage of this type of materials is that they can be completely biodegraded. However, because of the limits of the source of raw materials and the synthetic processes, these products are very expensive whose price is several times or even tens times of that of normal plastics. Furthermore, when they are processed into thin films, they have small elongation, and the products are very fragile, such as the products described in GB2058808 and U.S. Pat. No. 4,293,539. Therefore, they can only be suitable to special use. The other one is the blend of biodegradable natural products with synthetic resins, wherein the natural products provide the biodegradability, whereas the synthetic resins provide various mechanical and processing properties required during the application. The natural products often used are starch, cellulose and other polysaccharide and their derivatives. The synthetic resins can be polyethylene, polypropylene, polystyrene, polyvinyl chloride, etc. Starch is widely used because of its wide source, regeneration ability and low cost. The processes for the preparation of degradable starch resin using starch and synthetic resins can further be classified into three types: one is to prepare said degradable starch resins by using starch and hydrophilic synthetic resins, such as polyvinyl alcohol and copolymer of ethylene and ethenol, and other additives (for example WO9102023). Such resins prepared thereby have good biodegradability but poor water resistance, and the property of products is largely influenced by environmental moisture, the processing property is also poor; the second one is to prepare said degradable starch resins by treating dry starch with organosilicon compounds and titanate, converting starch from hydrophilic material into hydrophobic material, followed by blending with synthetic resins. This process is simple, and the processing techniques and equipments of the product can be those as used during the processing of synthetic resins. However, the starch content in final product is only in the range of 6-16%. In most cases, the starch content in the final products is below 10%. When starch content is high, the product could not be used because of the poor mechanical property, when starch content is low, the degradability is very poor; the third one is to prepare said degradable starch resins by mixing starch, polyethlene, mixing promoter (EAA) and plasticizer and the like, dextrinizing starch in the presence of outside water, blending it with polyethylene and EAA, removing additional water by extruding, and pelleting. The characteristic of this process is that the dextrinizing and blending of the starch is completed simultaneously and it makes possible to apply wet starch (for example EP 0409789A2). However, the starch content in the degradable starch resins produced by this process is not high, and when the starch content is higher than 25%, the mechanical property of the products is poor and the processing and application of the products are largely influenced by moisture. Thinner films could not be obtained when blowing, and the transparency is not good. It has been demonstrated by our study that the film obtained by such process has a thickness of above 50 .mu.m. It will soon become fragile upon the storage under dry environment, the elongation dropped abruptly, and the thermal fusion property is bad. It has been found under the observation of electronic microscope that the grain size of starch dispersed in polyethylene is in the range of 30-50.mu. and the dispersity is poor. Starch is a strong polar macromolecular material, and it has strong intermolecular and intramolecular hydrogen bonds, while polyethylene is non-polar polymer. Therefore, the critical point for this technique is to improve the compatibility between starch and polyvinyl resins. Furthermore, when such blend is manufactured into products, it is likely to appear pores during blowing as a result of the different fluidity of starch and synthetic resins. In prior art, it mainly use EAA as a mixing promoter. However, it is indicated by research that EAA is only partly compatible with starch and their mixing substantially belongs to physical mixing. Additionally, the applied water which is added to dextrinize starch actually functions to separate vinyl resin and starch. Therefore, it affects the dipersion of starch in polyvinyl resin. After the water content in starch is volatiled, the starch will recrystallize, making the grain size of dispersed starch too large which in turn affects the thickness and strength of the film and restrains the amount of starch.
In EP patents, starch with small grain size can be obtained by first reducing the grain size of starch through the action of microbes or enzymes, then emulsifying with vegetable oil, coating and spray drying. The amount of starch is therefore increased. But the process is very complicated. Actually among many techniques disclosed heretofore, when the starch content is higher than 30%, the mechanical property of the product is much worse as compared with normal plastics. Hence, it is very difficult to produce film products having practical value.
In addition, in prior art, the influence to the preparation of resin blend by the molecular property of polyethylene is often neglected, only its function of being the plastic material to provide thermal forming and mechanical property is emphasized.
As is described above, in prior art, biodegradable resin composition, especially degradable starch resin composition with low cost, good biodegradability and good applying property has not been obtained yet.